This invention is in the field of semiconductor integrated circuit manufacturing, and is more specifically directed to the formation and processing of dielectric films in semiconductor integrated circuits.
It is a fundamental goal, in the field of integrated circuit manufacturing, to design and manufacture integrated circuits to be as small as possible. As is fundamental in this field, the manufacturing cost of an integrated circuit corresponds strongly to the wafer area occupied by each integrated circuit die, not only by increasing the number of possible integrated circuits per manufactured wafer, but also by generally providing an increased theoretical yield for a given manufacturing defect density. In addition, the smaller device feature sizes involved in decreasing chip area also provide improved performance, and increased functionality per unit area.
Recent advances in the area of integrated circuit metallization technology have been important in decreasing the necessary chip area for modern integrated circuits. One such advance is the increased number of metal levels that are manufacturable in a device, providing both dramatic reduction in necessary chip area and corresponding dramatic increases in device density. Recent technological advances have also provided significant reductions in the line pitch of conductors in these multiple metal levels, also greatly increasing the functional density of the chip. The advent of copper metallization has also been important in providing reliable small line width conductors in modern integrated circuits. An example of a conventional copper damascene process is described in U.S. Pat. No. 6,410,426, assigned to Texas Instruments Incorporated and incorporated herein by this reference.
It is of course important to ensure good electrical isolation between adjacent metal conductors. By way of definition, the insulating material between conductors in the same metallization level is commonly referred to as the intermetal dielectric, or IMD, and the insulating material between conductors in adjacent metallization levels is referred to as the interlayer dielectric, or ILD. For performance and cost reasons, it is desirable to have adjacent conductors as close as possible to one another. This has necessitated the use of so-called “low-k” dielectric material for the insulator layers between metal conductors. Low-k dielectric materials refer to those insulating materials that have a dielectric constant lower than that of silicon dioxide. Because the capacitance between adjacent conductors depends on the dielectric constant of the insulating material that separates the conductors, as well as the thickness of this insulating material, a low-k dielectric material can be thinner than a higher-k dielectric material, while providing the same or better electrical isolation. The use of low-k dielectric materials is especially important in modern high-frequency integrated circuits.
Examples of modern low-k dielectric materials include fluorine-doped silicon dioxide (also referred to as fluorinated silicate glass, or FSG), organosilicate glass (OSG), thermoplastic organic polymers, aerogel, xerogel, and other conventional low-k insulator materials.
An important class of low-k dielectric material is referred to as substitution-group depleted silicon oxide. By way of definition, these materials have the chemical formula R1R2SiOx, where R1 and R2 refer to hydrogen, oxygen, a methyl group (CH3), an ethyl group (CH2CH3), phenyl, or a dangling bond. These materials provide a lower dielectric constant than stoichiometric silicon dioxide (SiO2).
Of these substitution-group depleted silicon oxide low-k dielectric materials, organosilicate glasses have become popular. Examples of commercially available OSG dielectric materials include the CORAL family of low-k dielectrics available from Novellus Systems, Inc., and MSQ low-k dielectric material available from JSR Corporation, of which LKD-5109 is an example. These materials are typically spun-on to the surface of the wafer being processed, and thus are also referred to as SOG, or spin-on glasses.
It has been observed, according to this invention, that these low-k dielectric materials are susceptible to damage from plasma processes that are performed after their deposition. Plasma etching of the dielectric film itself, or of other features, can damage the surface of the remaining low-k dielectric film. Plasma ash processes for removing masking material such as photoresist that defines the trench and via locations, also damage the exposed surfaces of the low-k dielectric. Plasma-enhanced chemical vapor deposition (PECVD) processes, such as for depositing a dielectric capping layer over the low-k dielectric material after trench and via etch, can also damage the low-k dielectric material.
In the case of OSG low-k dielectric materials, it has been discovered, in connection with this invention, that the plasma damage is caused by a chemical reaction between constituents of the material and the excited species in the plasma. Specifically, it is believed, in connection with this invention, that the OSG material is damaged by the conversion of silicon-hydrocarbon bonds in the material to silicon-hydroxide bonds when the material is exposed to oxidizing or reducing plasmas. Examples of these undesirable reactions will now be described in further detail relative to FIGS. 1a through 1d. 
FIG. 1a illustrates, in cross-section, a portion of a partially formed integrated circuit. Underlying structure 3 refers, in a general sense, to underlying structures and layers over which a subsequent metal conductor will be formed. As such, underlying structure 3 may include the underlying semiconductor substrate and any epitaxial layers, wells, and doped regions formed at a surface of the substrate, overlying insulator layers, conductive levels including polysilicon or metal gate layers and levels, and previous metal conductor levels, including refractory metals and copper or aluminum metallization. Low-k dielectric film 2 is disposed over underlying structure 3, and in this example is formed of an organosilicate glass. As such, in this example, silicon atoms are bound to three oxygen atoms and to one methyl group (CH3), corresponding to the organic content present in organosilicate glasses. As evident from FIG. 1a, dielectric film 2 has been etched, with photoresist mask 4 present at locations defined by photolithographic patterning. At the exposed locations, dielectric film 2 is etched through to underlying structure 3; alternatively, for example in the formation of a conductor trench, dielectric film 2 may only be partially etched through at this location.
FIG. 1b illustrates the reaction of the OSG resulting from a subsequent oxidizing plasma, such as used to remove the remaining portions of photoresist 4 from the surface of dielectric film 2. The plasma illustrated in FIG. 1b is an oxygen plasma, as conventionally used in the art for photoresist removal. As evident from FIG. 1b, the methyl group in molecules at the exposed sidewalls of dielectric film 2 has been replaced by hydroxyl (OH) group, as a result of the oxidizing plasma exposure. In effect, the bonds between the silicon atoms and the methyl groups in these surface molecules are lost or broken, with the hydroxyl group replacing the lost methyl groups. The Si—OH substituted moieties are referred to as “silanol” moieties.
FIG. 1c illustrates the undesired reaction of surface molecules in MSQ OSG dielectric film 2 in the presence of a reducing plasma, such as a hydrogen plasma. In this instance, the excited neutral and/or ionic hydrogen species also react with and displace the methyl group in OSG materials near the exposed surfaces of dielectric film 2, for example along the sidewalls of the etched trench or via as shown. The bonds are either left dangling (i.e., associated with an unpaired electron spatially localized at the site of the removed methyl group) in this instance, or a hydrogen atom attaches to the silicon atom in the place of the removed methyl group. After exposure to the reducing plasma, dielectric film 2 becomes vulnerable to additional reaction with moisture, as evident from FIG. 1d. Water molecules react with the modified dielectric moieties, attaching hydroxyl groups either to the dangling bonds at the surface of dielectric film 2, or replacing the hydrogen molecules that bonded to the silicon atom after the hydrogen plasma exposure, in either case forming silanol molecules. The overall reaction in this situation follows: 
In the case of exposure to either hydrogen-containing or oxygen-containing plasmas, the Si—OH bonds at the surface of the OSG material have been observed, in connection with this invention, to degrade the integrity of low-k dielectric film 2. One form of degradation is the increase in the dielectric constant of the low-k dielectric material due to the presence of the silanol. In addition, the damaged OSG material has been observed to adsorb moisture. It has also been observed, in connection with the invention, that this degraded low-k dielectric material is vulnerable to chemical attack during exposure to wet chemical cleanups, which results in significant critical dimension (CD) loss of low-k dielectric film insulating structures.
As mentioned above, thermal annealing of the low-k dielectric film to negate already-occurred damage from plasma processing is known in the art. In connection with this invention, it is known that such thermal annealing removes physically adsorbed, but unreacted, moisture present at the surface of the low-k dielectric film. However, it has also been discovered, in connection with this invention, that the thermal activation of these silanol condensation reactions necessitates annealing temperatures in excess of 250° C. for prolonged periods of time such as on the order of 103 to 104 seconds, or annealing temperatures in excess of 400° C. for brief durations such as on the order of 102 to 103 seconds. These temperatures and processing times are both technically and economically unfavorable, due to concerns of activating other thermal processes such as copper stress migration. Significant equipment costs and increases in manufacturing times are also involved in such annealing. In addition, plasma-damaged low-k dielectric films that are annealed according to conventional processes, while removing the physically adsorbed moisture, are vulnerable to the re-adsorption of moisture and subsequent reaction which result in the formation of silanol molecules in the low-k dielectrics and the corresponding degradation of these films.
Similar effects are believed to occur in other substitution group depleted silicon oxide low-k dielectric materials, in which the bonds between the silicon atoms and the substitution groups are converted to silanol when exposed to oxidizing or reducing plasmas.
By way of further background, the modification of the bulk properties of hydrogen silsesquioxane (HSQ) with ammonia is known. This conventional treatment is for the purpose of avoiding the necessity of forming a barrier or liner layer for the subsequent deposition of copper metal over the HSQ film.